Note. Colored lines indicate individual participants. Black lines indicate the predicted interaction effect based on the mixed effect model depicted in Table 6. The waitlist period occurred between sessions W1 and T1, while the heart rate variability biofeedback intervention period took place between T1 and T5. RMSSD – root mean square of successive differences; ns – not significant.
 

3.6         Attentional control

The reaction time prediction model included main effects for TIME, TREATMENT, CUE, and FLANKER. Additionally, two three-way interactions were included – TREATMENT * TIME * CUE and TREATMENT * TIME * FLANKER. Both of these three-way interactions revealed significant terms in the model (see Table 7).
 
Reaction Time ~ treatment:time:cue + treatment:time:flanker + time + treatment
                                     + cue + flanker + (1|vpn) + (1|item).
 
We used linear trend estimates from the emmeans package (v1.8.8) to compare valid vs. invalid (cue) and congruent vs. incongruent (flanker) slopes in each pre-post comparison. This approximated the original score calculations. Significant differences in slopes indicate changes in the Orienting Score (invalid vs. valid cue trials) or Executive Score (incongruent vs. congruent flanker trials) between pre and post-assessment.
In the TIME * TREATMENT * CUE interaction, there were no significant differences in pre-post slopes between valid and invalid trials during both the intervention period, d = -0.03, zratio = -0.67, p = .91, and the waitlist period, d = 0.00, zratio = 0.03, p = 1, (see figure 7a).
In the TIME * TREATMENT * FLANKER interaction, there was a significant difference in pre-post slopes between congruent and incongruent trials during the intervention period, d = -0.18, zratio = -4.60, p < .001, but no difference in the waitlist period, d = -0.06, zratio = -1.34, p = .54. The effect in the intervention period indicated less difference between valid and invalid trials post-treatment compared to pre-treatment, equivalent to better Executive Scores (see figure 7b).
 
Figure 7. Course of attentional control